Single side band diversity radio receiving system



/A/ VEA/TOR G HORN A T Tom/Ey F. A. F'OLKINGl-lom SINGLE SIDE BAND DIVERSITY RADIO RECEIVING SYSTEM med Aug. 2s, 1959 Oct. 29, 1940.

85,4.POLK/N WNW Patented Oct. 29, 1940 UNiTED STATES SINGLE SIDE BAND DIVERSITY RADIO RECEIVING SYSTEM Frank A. Polkinghorn, Bloomfield, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 23,

l 10 Claims.

This invention relates to radio communication systems, and more particularly to methods and means for reducing fading effects and maintaining a favorable signal-to-noise ratio in a diversity reception system.

In a diversity system the same transmitted signal is simultaneously received from the signal source at the transmitting station over a plurality of paths through space. The amplitude of received signal over the various paths usually varies as between the different paths with changes in the condition of the space transmission medium, so that as the amplitude of the signal received over one path increases, that over another or other of the paths may diminish. These amplitude variations are generally at random and are continually taking place.

It is customary in certain types o-f diversity systems either to provide means for automatically switching the signal receiving apparatus at the receiving station so as to keep it connected with the path over Which the signal of the greatest amplitude at any particular time is arriving, or, alternatively, to permit the signal receiving apparatus at all times to be connected with all of the paths and to automatically control the energy contributions to the signal receiver by the various paths in proportion to the varying amplitudes of the signals being received over them. It is toward diversity operation of the latter sort that the present invention is particularly directed.

In the double side band type of radio transmission the carrier upon Which the side bands are based is usually transmitted with the side bands at sufficient amplitude to provide for the proper demodulaticn of the side bands in the nal detector at the receiving station. When general fading takes place, the carrier fades With the side bands; and as the noise introduced at the receiving station input of the fading path yields in the receiving apparatus an audible component by interaction With the unmodulated carrier frequency Wave component as does a modulation of the carrier frequency, the same fading of the carrier and its side bands which reduces the signal amplitude also reduces the noise amplitude.

In the single side band type of radio transmission it has been the practice either to eliminate the carrier entirely, or totransmit it at a much reduced energy level. When no carrier is transmitted, a suitable demodulating carrier of the proper frequency is generated and supplied at the receiving station. When carrier is transmitted at a reducedenergy level, it is customary either to utilize it at the receiving station to 1939, Serial No. 291,481

(Cl. Z50- 20) control the frequency of a local oscillator that supplies the frequency for final demodulation, or, after reconditioning and amplifying at the receiving'station, to utilize it at the proper fre- M quency directly to effect the demodulatio-n of the "f5 signal side bands. In either case" the locally generated demodulating frequency,'=or the demodulating frequency controlled 'by the original carrier, or the reconditioned and amplified original l, A n

carrier itself at the proper frequency for demodulation has in some casesbeen supplied to the final Adetectors at a xed amplitudethat does not vary with the variation of carrier amplitude that may take place as a result of thepresence of fading over the space path.

Such practice is not favorable to the bestoperation of a diversity system in which the plurality of paths over which the signal is received are at all times connected with thesignal receiving apparatus at the receiving station. This is be-v cause the fading over any path which diminishes the amplitude of the signal does not diminish the amplitude of the noise that entersv the receiving system by Way of the receiver input of that path. The constant amplitude of the locally `25 supplied carrier, or of the reconditioned received carrier, causes the relative noise output of that path to remain constant even though the signal contributed by the Ipath has faded. Therefore all of the paths contribute noise to the common signal receiving apparatus,` lWhile only those of them in which fading has not foccurred contribute signal at a favorable signal-to-noi'se ratio. The result is that, in the absenceA of some corrective measure, the over-all signal-tonoise ratio of such Fw a system is adversely affected.

In the diversity systeml of the present invention this effect is overcome bythe employment of a method' and means for causing the amplitude of the demodulated signal and accompanying '40 noise contributed byl each kof the diversity branches of the receiving system to vary With the amplitude, and under the control, vof a selected portion of the energy freceived from the u transmitting station over the corresponding space 45 path. Specifically, in the preferred embodiment, when the signal and selected controlling portion, which may for example bethe accompanying carrier or an accompanying pilot frequency, of

any path fade, the amplitude of the demodulating frequency locally supplied to the corresponding diversity branch islikewise diminished; and as both the signal output andthenoise output of the branch depend upon the amplitude of the locally supplied carrier or demodulating frequency, the demodulation being non-linear in character, the noise output is diminished with the signal output. Therefore, a favorable over-all signal-to-noise ratio in the signal receiver is maintained at the receiving station.

The manner in which this result is accomplished will be explained by reference to the accompanying drawing which represents schematically a single side band diversity receiving system consisting, as illustrated, of two radio reception branches, each arranged to receive radio signals arriving from the same signal source over different space paths, convert' the signals to audio frequencies and transmit them nal receiving device. 4 A

For the sake of simplicity, communication of the radio receiving apparatus-with Vthe different space paths is represented as being yby vvay of the antennas AI and A2, which may be of any suitable type. These antennas feed over radio-recaption-branches I and 2, respectively, .to the common signal.receiver I0. Only 'two antennas with their associated radio receiversare shown, but it lWill .b e understoodthat any greater number,

usually three in a space diversity system, may be used. The elements included in each radio reception. branch are of identically the same type as thoseincluded inthe other branch; and corresponding elements aredesignated by the same reference numbers, excepting that a prime is affixed -to the designating numerals of the second branch. The description of the arrangement and operation-of. the elements in one branch may therefore be understood to apply to the operation and arrangement of the elements in the other branch.

jTThe energy-received by the antenna passes to high frequency amplifier and first detector elements II.- In the first detector the received enffergy is modulated by a `high frequency Wave sup- .pIiedQby-t'he fbeat oscillator `2i) and the desired products of `modulation are-amplified in a twostage intermediate-frequency amplifier I2. The above-mentioned elements may be regarded as constituting the radiorreceiver of the reception branch Thel intermediate frequency amplifier 5I2- operates into .a'branched circuit, one branch of which includes anV isolation amplifier I3 and a non-linear second detector I4, which may be of the ordinary square-jaw type and the other ybranch of which selects a narrow frequency-band including the carrier that passes through the bandpass crystal filterll5. The carrier output from the filter vI5 is supplied through the transformer 28 to the grid circuits of the triode detectors I6 and I1 in parallel. These detectors operate'to rectify the carrier oscillations supplied thereto and produce direct current potentials proportional tothe amplitude of such carrier os- "cillationsl To the output-of thesecond detector I4 there is supplied 'a local carrier frequency from the local car-rier oscillator 3K0. 'I'his local carrier is ampliiied in the local carrier amplifier I8 and supplied throughthe transformer 29 to the plate circuit ofthe detector I4. The degree of amplificationof the carrier is controlled by the potential across` the resistance I9 included in the output circuit .of the detector I6, this potential being appliedl in opposition to the normal biasing potential from the battery 22 in the input circuit of the amplifier I8, as indicated by the polarity markings onresistance I9 and battery 22. The carrier oscillations `supplied to the detector M of branch to a common sig'-l 2 are similarly controlled by the amplifier I 8' and its associated circuits.

The signal frequencies which result from the demodulation that occurs in the second detector I4 of branch I, together With those that result from the demodulation in the second detector I4 of branch 2, are inductively communicated by Way of transformers 2| and 2|', respectively, to the input circuit of the audio frequency amplifier 50 Where they are combined, amplified, and passed to the common signal receiver I0.

'Ihe detector tubes I'I and I'I of branch I and branch 2, respectively, control the potential drop across the resistance 40 which is common to the plate circuits of the detectors I'I and I'I. The variable potential thus developed across resistance `4I] is applied to the amplifiers I2 and I2 of branch I and branch 2, respectively, to give the usual ,diversity common automatic volume control to the system. 'I'he value of this variable control-potential is determined by the combined strength of the carrier Waves received in the two branches I and 2 as the input of the detectors II yand I'I' is ,connected to the respective carrier lilters I 5 and I5 as described above.

In the operationof the system it is assumed that the distant station is transmitting a radio frequency carrier of relatively small amplitude, and a single side band resulting from the modulation of the carrier frequency with the signal frequency.- The radio receiver elements II and II of the two radio reception branches I and 2 are tuned to select the carrier and its side band, and the beatfoscillator 20 is so adjusted as to produce, by modulation with the incoming wave,

an intermediate frequency Wave of the desired frequency, for instance, 400 kilocycles. A portion .of this amplified intermediate frequency, as has been described, passes into the branch circuit in.. cluding the band-pass crystal filter I5. This lfilter'may be designed to pass a frequency band Vtransmitting station and in the beat oscillator 20 at the receiving station. For a moderate degree Aof frequencyinstability in these oscillators the band passed by the filter I5 may be 200` cycles wide, or narrower to the extent that the oscillavtor stability and rate of change of carrier or pilot frequency permits. After passing through filter I5 the narrow band including the carrier at its intermediate frequency value passes to the input .circuits of detectors I6 and II.

In the output of detector I6 there is produced rafdirect current, the amplitude of which varies with the variations in the amplitude of the carrier received over the corresponding branch. Any fading of the carrier and its associated side .band at the receiving antenna of the corresponding branch results in a reduction in current flowing through the resistance I9 in the output circuit of detector I6 and a corresponding potential reduction across the resistance.

'I'he tube of the local carrier amplifier I 8 is preferably ofthe variable mu type. The bias in the input circuit of this tube is determined by the potential of the biasing battery 22 and the potential drop across the resistance I9. This bias is so adjusted that with no carrier input to detector .I6, amplifier I8 gives very little local carrier output to secondV detector I4. With a normal carrier input` to detector I6 the increased potential across resistance I9, in opposition to the potential of biasing battery-22,'decre'ases the negative bias in the input circuit of amplifier I8 'to-produce normal gain in the local carrier frequency supplied to second detector I4. Thus, the amplitude of the signal contributed bythe non-linear second detector of each receiving branch of the system is automatically varied with the variation of the radio frequency input to the corresponding branch, and when the signal contribution of any of the branches is diminished by fading', the noise contribution of that branch is diminished with it. A

As indicated in the drawing, the amplifier I3 in the signal subbranch of each of the branches I and 2 is of the screen grid type for preventing the local carrier oscillations which are fed into the plate circuit of the second detector I4 by the amplier I8 from feeding back through the channel and appearing in the other detector circuits.

The diversity common automatic volume control which is secured by employing the voltage drop across the resistance 40 common to the plate circuits of the detectors I'I and I'l' to control the grid bias of the amplifiers of several branches simultaneously and equally, is well known in the art and needs no specificv description. It ensures that the inputs to the detectors of the several branches shall have the same relative signal strengths as the respective branch inputs. l i

If it be assumed that, from a condition of equality, the radio frequency energy received upon antenna AI and transmitted through radio reception branch I increases, while that received upon antenna A2 and transmitted through radio reception branch 2 diminishes, the common automatic volume control eiected by detectors I'I and I'I and the individual transmission gain control eifected by detectors I6 and I6' will operate in such a way as to tend to magnify the co-ntribution of branch I which has the more favorable signal-to-noise ratio and to reduce the contributionof branch 2 which has the less favorable signal-to-noiseratio. 'Ihe increasing negative bias on the amplifier tubes of both branches resulting from the increasing radio frequency input from antenna AI stabilizes the volume contribution of branch I at a predetermined value and reduces the relative contribution of branch 2 in which the signal is fading, At the same time detector I6 of branch I operates to reduce the negative bias in the input circuit of amplifier I8 to permit a proportionately greater local carrier input to the output circuit of secondk detector I4` of branch I; while detector I6 of branch 2, responding to-the diminishing energy input received from antenna A2, reduces the opposing biasydeveloped in resistance I9', thus increasing the negative bias in the input circuit of amplifier I8 to diminish the output of local carrier to second detector I4 in accordance with the reduction in radio frequency energy received over the corresponding branch. The result is that the signal received in the common signal receiver I0 is principally that received over the path of the diversity system which has the most favorable signal-to-noiseratio, while the noise that would. otherwise enter the common receiver over the path or paths of reduced'signal energy is diminished by the device which reduces the gain of the transmission path in accordance with the reduction in the received radio frequency energy. In a properly proportioned system in accordance with the present invention the signal volume delivered to signal receiver I0 is held substantially constant. l

If it should be desired to effect a greater degree of frequency stability, this'may be done inthe manner disclosed in Patent 2,041,855 to R. S. Ohl, fl issued May 26, 1936. That is, the filtered carrier received from the transmitting `station may be reconditioned and then used for obtaining automatic frequency control of the beating oscillator k2l! and synchronization of the local carrier oscillator'3Il. Or, if desired, in accordance with'the disclosure of the Ohl patent, the filtered carrier after being reconditioned and amplified may constitute the source of locally applied carrier oscillations in place of the local carrier oscillator 30. As disclosed in the Ohl patent,l the reconditioned carrier is supplied for demodulation' atsubstantially constant amplitude regardless of fading in transmission, and therefore, if applied in the ,l

present system in substitution of the carrier oscillations generated locally by oscillator 3G, the system is susceptible of and requires the same automatic amplitude control in accordance with the amplitude of the rectified energy of the branch `to which it is applied that is utilized in the present invention. l

'Ihe specific disclosure herein is that of a diversity system in which the same signal is simultaneously received over a plurality of paths by way of separate antennas. It will be understood that `the invention is also applicable to an angle diversity system -in which 'the same signal is simultaneously received at ydifferent vertical angles by way of a single array of spaced antenna units and is demodulated at each angle by reconditioned carrier arriving at that angle, or by locally generated carrier synchronized the received carrier of the corresponding path.

hij.:

In the foregoing the energy received over each branch and selected and rectified to control the slgnal transmission gain of that branch,l has been specifically described as that of the transmitted carrier frequency. It will be understood that in systems where the carrier is entirely-suppressed and a. pilot frequency is transmitted in its place, as a side band of the suppressed carrier, this pilot frequency may be selected and rectified and used to control the signal gain of the corresponding branch inthe same manner as has been described for the carrier. l

What is claimed is: v 1.. In a diversity radio receiving system, a plurality of radio receivers each arranged to receive radio; signals arriving from the same signal source L by different paths, a detector and signal frequency amplifier system for each receiver, a signal circuit responsive to the combined signal outputs of said systems, means for selecting and rectifying a portion of the energy received from the transmitting station by each receiver; means for combining the rectified currents, means for controlling the amplification of all the receivers in accordance with the combined rectiied currents so that the inputs to the respective detectors have the same relative signal strengths as the inputs to the receivers, means for supplying .carrier energy to the detector and signal frequency amplier systems, and means` individual to said receivers and responsive to the individual lrectified Qurrents for so controlling the conversion gain of the individual detector and signal frequency amplifier systems as to'rnaintain the individual signal outputs .of said receivers proportional to the inrality'of radio receivers, each arranged to receive signals arriving from the same signal'source by dilerent paths, each receiver, having a detector, a signal circuit responsive to the combined signal outputs of said receivers, means for. selecting and rectifying a portion of the energyreceived from the transmitting station by each receiver, means for combining the rectified currents, means `for controlling-the amplication of all the receivers in accordance with the combined rectified currents so that the inputs to the respective detectors have the same relative signal strengths as the inputs to the receivers, a source of carrier oscillations, paths for supplying oscillations from said source to said detectors, and means for controlling the transmission oi said oscillation paths in response to the individual rectied currents to maintain constant the ratio of signal to carrier voltages on each detector.

3. In a diversity radio receiving system comprising a plurality of radio reception branches each connected with a common signal receiver and each including a signal detector, means operatively coupled to each branch for selecting and rectifying the signal carrier, means responsive to the rectified carrier current of at least one o'f the branches for similarly controlling the amplication of all of the branches so that the inputs to the respective detectors have the same relative signal strengths as thev signal strengths at the respective branch inputs,` a source oi' substantially constant amplitude carrier oscillations at the receiving station, means for supplying carrier oscil- I lations from said source to the detectors of'the various branches, and meansresponsiveto the amplitude of the rectified carrier current of each individual branch for-.controlling the' amplitude of carrier oscillations supplied from said source to the detector of the corresponding branch.

4. In -a diversity radio receiving system comprising a plurality of radio reception branches each connected With a common signal receiver and each including a signal detector, means operatively coupled to each branch for'selecting zand rectifying the signal carrier, means for combining the rectied carrier current of all the branches tosmilarly control the'amplii-lcation of all of the branches so that the 'inputs to the respective detectors have the same relative signal strengths as the signal strengthsat the respective branch inputs, a source of substantially constant amplitude carrier oscillations, means for supplying carrier, oscillations from said source to the detectors of the various branches,` and `means responsive to the amplitude 'of the rectified carrier current of each individual branch for controlling the amplitude of the output supplied to the common signal receiver by the detector of the corresponding branch.

5. In a single side band diversity radio receiving system comprising a plurality of radio reception branches connected with 'a common signal receiver and each including a signal. detector, means operatively coupled to each branch for selecting and rectiiying a portion of the energy received from the transmitting stationby way of the corresponding branch, a source of substantially constant amplitude Acarrier oscillations, variable transmission means for supplying oscillations from said source to the detectors of the various branches, and means responsive to the rectified energy of each branchfor controlling said transmission means to regulate the amplitude of oscillations applied to eachdetector from said .sourceinaccordance with' the amplitude of said rectified energy-of the corresponding branch.

. 6. In @a diversityl radio receiving system comprising a 4plurality of radio reception branches connected with a common signal receiver and -eachV including a signal detector, means operatively coupled to each branch for selecting and rectifying a portion of the energy received from the transmitting station by Way of the corresponding-branch, means for combining the rectifiedxenergy. derived from all of the branches, means responsive to said combined rectified energy for similarly controlling the amplification of all of the branches so that the inputs to the respective detectors have the same relative signal strengths as the signalsat the corresponding branchY inputs, and means responsive to the rectied-energyof each branch for controlling the amplitude of the detected signal output of the corresponding branch in accordance with the amplitude of said rectified energy of the corresponding branch.

7. In a single side band diversity radio receiving system comprising a plurality of radio reception branches connected Witha common signal receiver and each including a signal detector, means operatively coupled to each branch for selecting and rectifying a portion ofthe energy received from the transmitting Vstation-by way of the corresponding branch, means responsive to the rected energy derived from at least one of the branches for similarly controlling the amplication of all of the branches so that the inputs to the respective detectors have the same relative signal strengths as the signals at the corresponding branch inputs, a source of substantially constant amplitude carrieroscillations, means for supplying oscillations from said source to the detectors ofv the various branches, and means responsive to the rectied energy ofY each branch for controlling the amplitude ofthe signal output of the corresponding branch in accordance with the amplitude of the said rectiiied energy of the corresponding branch. Y

8. The method of maintaining a favorable signal-to-noise ratio in a diversity radio receiving system utilizing means for receiving in different radio reception branches the radio signals arriving from the same signal source over diilerent space paths, applying equal amplification to the energy received in each branch, separately selectingand rectifying a portion of the energy received over each branch, and separately demodulating the signal energy component of each branch by carrier oscillations supplied from a substantially constant ,amplitude local source, which consists in utilizing the rectied portion of the energy of each branch to control in accordance with its own amplitude the amplitude of the demodulated signal component of the corresponding branch, and combining and utilizing the demodulated signal components of all the branches.

9. 'I'he method of maintaining a favorable signal-to-noise ratio in a diversity radio receiving system utilizing means for receiving in different radio reception branches the radio signals arriving from the same signal source over different space paths, applying equal amplication to the energy received in each branch, separately selecting and rectifying a portion of the energy received over each branch, and separately demodulating the signal energy component of each branch by carrier oscillations supplied from a substantially constant amplitude local source, which consists in utilizing the rectied carrier component of each branch to control in accordance with its own amplitude the amplitude of the demodulating carrier oscillations supplied to the corresponding branch from the local source, and combiningand utilizing the demodulated signal components of all the branches.

10. The method of maintaining a favorable sig-` nal-to-noise ratio in a diversity radio receiving system having a plurality of -radio reception branches connected with a common signal receiver and arranged to receive radio signals arriving from the same signal source over diiierent paths, which consists in utilizing the combined carrier energy received over the diierent branches for similarly controlling the radio frequency amplication of all of the branches, and utilizing the carrier energy received over each individual branch for controlling in accordance with its amplitude the amplitude of the signal contribution of the corresponding branch to the common signal receiver.

FRANK A. POLKINGHORN. 

